MEMS transistor integrated on CMOS

PORTLAND, Ore.—A micro-electro-mechanical system (MEMS) transistor was announced by Semiconductor Research Corp. (SRC) and Cornell University, giving SRC members access to on-chip timing solutions for their CMOS chips.

The MEMS-JFET builds a junction field-effect transistor (JFET) on top of a silicon resonator, providing both amplification and a rock-solid mechanical reference for on-chip channel-select filters and oscillators.

"We believe that this research result will allow the direct integration of radio frequency sources onto silicon chips alongside all the other CMOS circuitry," said Kwok Ng, senior director of device sciences at SRC.

The process works by integrating the MEMS and JFET structures on a silicon-on-insulator (SoI) substrate using conventional CMOS processes. Sacrificial oxides are then etched from beneath the single-crystal silicon resonator leaving it suspended. Using a p-n junctions as the transducer, the JFET can be made to oscillate at a frequency determined by the dimensions of the suspended MEMS resonator.

Junction field-effect transistor (JFET) built on a silicon resonator provides both amplification and a rock-solid mechanical reference for on-chip channel-select filters and oscillators.

The resulting timing circuits using the MEMS-JFET should enable the integration of oscillators and filters right onto CMOS chips alongside their other circuitry, rather than require separate quartz, CMOS or MEMS oscillator chips, as is the case today. The new integrated process also offers higher quality factors, achieves higher efficiency than conventional MEMS resonators, and can operate at gigahertz frequencies, according to the researchers. The prototype operated at 1.61GHz with a quality factor of 25,900 at room temperature.

Since the device does not require a separate transducer material, the team claims superior temperature stability than conventional MEMS. Also, since the techniques uses an active JFET as an amplifier, the researchers maintain that it achieves lower phase noise, as a result lower flicker noise, than today's MEMS oscillators.

Funding was provided by SRC's Global Research Collaboration and Focus Center Research Program Center for Materials, Structures and Devices. SRC credits previous related work on reducing transconductance-to-bias current ratio at Cornell, MIT, EPFL and CNRS.

1.6 Ghz sounds impressive but it is likely the speed of transistor (bandwidth of the signal that passes thru it) not its switching frequency which is likely much lower, any idea how low it is? It used to be that these MEMs switches are only good for configuration purposes, you can't switch anything in real time...Kris

I received this comment from Sunil Bhave, principal investigator from Cornell University:
"Hello, the device is not a switch, its a resonator. The center frequency of the device is 1.61GHz. and it has a Quality Factor of ~26,000. The bandwidth of the JFET that is integrated with the resonator is in the 10s of GHz range, so there is no bandwidth concerns with the pickoff mechanism"